Mauna Loa is inflating again Oblique shaded-relief map of the five volcanoes that built the Island of Hawai`i. Mauna Loa encompasses 51 percent of the island's surface area; its most recent eruption was in 1984. Click for large map. Continuously recording GPS receivers were installed on the summit and upper flanks of Mauna Loa in 1999 to detect changes in shape of the volcano that typically accompany underground movement of magma. The GPS instruments allow such ground deformation to be tracked continuously and also more accurately than was possible before. Until May of this year, the new instruments showed what we already knew from other surveying techniques—the summit caldera was slowly subsiding and shrinking. But in late April to late May, centered on May 12 (Mother's Day), the GPS receivers began moving slightly in a different direction. Since May, the instruments show that Mauna Loa's summit area is rising slightly and the caldera is widening 5-6 cm per year. What caused the abrupt change? Our computer models of the moving GPS receivers suggest that the current ground motion is the result of magma rising into a reservoir about 5 km below the caldera. Only time will tell if this new trend will continue in the coming months. The nature of seismicity changed several months before each of the latest two eruptions. Large increases were obvious in both the numbers of earthquakes and the amount of energy released by these earthquakes. That is a reasonable expectation before the next eruption, though we must be cautious in saying that such an increase must occur. For more information, see: Current activity of Mauna Loa Summary of monitoring data (1974-September 2002)

Archive of previous feature stories